1. Field of the Invention
The present invention relates generally to optical communications, and more specifically to optical fibers suitable for the fabrication of fiber Bragg gratings.
2. Technical Background
A high performance optical telecommunication system carries high data rates over long distances with no electronic regeneration. For example, rates of 10 Gb/s or more over unregenerated distances of three to five hundred kilometers have been achieved. A high performance system may employ high power signal lasers, optical amplifiers, dispersion compensation devices, optical switching devices, and may use wavelength division multiplexing. Optical telecommunications systems are progressing toward higher speeds and longer span lengths, making the requirements for system components more and more arduous.
One such system component is a fiber Bragg grating. A fiber Bragg grating is formed from a periodic modulation of the refractive index of the core of an optical fiber. Fiber Bragg gratings act to selectively retroreflect a single wavelength from a band of wavelengths propagating in an optical fiber. Fiber Bragg gratings have found utility in diverse applications such as laser stabilization, wavelength division multiplexing, gain flattening of amplifiers, and dispersion compensation.
Fiber Bragg gratings are conventionally fabricated by exposing an optical fiber with a photosensitive core to a pattern of UV radiation having a desired intensity modulation. The pattern of the UV radiation is generally formed using interferometric techniques, such as by passing the radiation through a phase mask. In order to fabricate an effective grating, it is desirable to have an optical fiber having a high photosensitivity. Conventional photosensitive optical fibers have a relatively high concentration of germania in their cores. While increasing the germania content acts to increase the photosensitivity of the core, it also acts to increase the refractive index of the core, and therefore the Δ and the numerical aperture of the optical fiber. Optical fibers having high Δ and numerical aperture tend not to couple well to standard single mode optical fibers. As such, fiber Bragg gratings written in conventional optical fibers having a high germania content in the core may have high coupling losses to other optical fibers, and therefore be disadvantaged for use in optical telecommunications systems.
Another method of increasing the photosensitivity of a germania-containing optical fiber is to load the optical fiber with molecular hydrogen under conditions of high pressure. While hydrogen loading is a useful method in the fabrication of fiber Bragg gratings, it adds the extra process steps of hydrogen loading and post-exposure annealing and requires the use of high pressures of a highly flammable gas.
Conventional photosensitive optical fibers do not provide for the manufacture of optical fiber Bragg gratings with the desired performance and simplicity of manufacture. There remains a need for an optical fiber that exhibits high photosensitivity and desirably low numerical aperture. From the cost and process point of view, it is further to have a photosensitive optical fiber that may be used in the manufacture of fiber Bragg gratings without hydrogen loading.